Document 1 (JP 2004-45304) discloses a distance measuring device based on the time of flight measurement. The distance measuring device disclosed in Document 1 is configured to emit intensity-modulated light having an intensity oscillating at a constant period such as a sine wave. The distance measuring device is configured to measure a phase difference between a waveform of the intensity-modulated light at emitted time and a waveform of the intensity-modulated light at received time, and calculate a distance to a target on the basis of the measured phase difference.
In the distance measuring device disclosed in Document 1, the phase difference between the waveforms at the emitted time and the received time is corresponding to a time period (time difference between the emitted time and the received time) starting at the time of emitting the intensity-modulated light and ending at the time of receiving the intensity modulated light reflected by the target. The time difference τ is expressed by the equation: τ=T(Ψ/2π), wherein T [s] is the period of the intensity-modulated light and c [m/s] is the light speed and Ψ [rad] is the phase difference between the waveforms. The distance L to the target is expressed by the equation: L=(1/2)c T(Ψ/2π).
The time measuring device disclosed in Document 1 has the maximum measuring distance which is corresponding to a half period of the intensity-modulated light. Therefore, the maximum measuring distance can be extended with a decrease in the frequency of the intensity-modulated light. However, decreasing the frequency of the intensity-modulated light for extending the maximum measuring distance causes a decrease in distance resolution.